1. Field of the Invention
The present invention relates to a driving technology for a plasma display panel, which divides each field of a video signal into a plurality of subfields, and displays multi-grayscale images by a combination of the subfields.
2. Description of the Related Art
A plasma display has a display panel having a plurality of discharge cells, in which a fluorescent layer is coated respectively, and which are arrayed in a matrix. Generally a display panel has a plurality of row electrode pairs which are formed on a substrate, a plurality of column electrodes which are formed facing the row electrode pairs, and a plurality of discharge cells formed at areas where the row electrode pairs and the column electrodes cross respectively. These discharge cells are arranged in a matrix, and a fluorescent layer is coated inside each discharge cell. In a plasma display, a gas discharge for initially adjusting the charge distribution in all the discharge cells (that is, a reset discharge) is executed first when an image is displayed. Then the plasma display generates a gas discharge in selected cells, out of the discharge cells (that is, an address discharge), and generates such charged particles as electrons and ions (that is, wall charges) so as to set the wall charge distribution in the selected cells to an emission enable state (that is light ON mode). Also a single or plurality of voltage pulses (that is, discharge sustaining pulses) are applied between the row electrodes constituting each row electrode pair, whereby the gas discharge is generated in the discharge cells in the emission enable state (that is a sustaining discharge). As a result, ultraviolet generated by the sustaining discharge excites the fluorescent layer, and allows light to be emitted. Multi-grayscale images can be displayed by controlling the number of times gas discharges, which are generated in the discharge cells per unit time.
A subfield method is normally used for a grayscale control method for a plasma display, dividing each field corresponding to one frame image into a plurality of subfields, assigning the weight of brightness, which is in proportion to an emission period, to each subfield, and displaying multi-grayscale images based on the combination of these subfields. The subfields are sequentially displayed along a time axis, so human eyes can perceive these subfields as one image by integrating the emission patterns. For example, if the weights of brightness to be assigned to 8 subfields constituting each field are set to the ratio of 20:21:22:23:24:25:26:27 (=1:2:4:8:16:32:64:128), then 256 grayscales of images can be displayed by combining the subfields. This type of grayscale control technology based on the subfield method is disclosed, for example, in Japanese Patent Application Laid-Open (Kokai) No. 2003-29698 and its corresponding US Patent Application Publication No. 2003/011543.
According to the grayscale control based on the subfield method, a reset discharge, for initially adjusting the charge distribution in all the discharge cells, is executed first in the display period of the first subfield out of the subfields constituting each field. However, light generated by the reset discharge (background emission) drops the contrast, particularly the dark room contrast, of the display image, and deteriorates the image quality. Here “dark room contrast” is normally defined as the ratio (=Lg/Lb) of the emission brightness (=Lg) when a white level image is displayed and the background emission brightness (=Lb) when a black level image is displayed. Dark room contrast is one parameter which determines the level of image quality, particularly when a low brightness image is displayed.